Many chemical manufacture and refining processes involve a difficult separation of a desired product compound from a fluid mixture containing one or more other compounds having physical properties in close proximity to those of the product compound. Examples of such difficult separations include separation of alkenes, such as ethylene or propylene, from the corresponding alkane, such as ethane or propane, and separation of styrene monomer from ethyl benzene. Currently, these difficult separations are typically performed by distillation. Distillation separation of such compounds, however, requires tall distillation towers and extensive heating to make a satisfactory separation, due to the close proximity of boiling points of the compounds being separated.
These difficult distillation separations consume a significant quantity of energy. For example, separation of alkenes from alkanes by distillation has been estimated to consume 0.12 quad per year of energy (1 quad equals one trillion billion BTU). This large energy consumption is largely due to distillation of the light olefins ethylene and propylene, which are two of the largest volume chemicals produced worldwide.
One method that has been proposed for separating olefins in a manner to avoid the high energy consumption of distillation is to make the separation with a facilitated transport membrane. A facilitated transport membrane is a thin film membrane that includes a carrier specie in the membrane that preferentially chemically interacts with a desired component to facilitate transport of that component across the membrane, thereby separating the desired component from an undesired component. In that regard, it is known that many alkenes undergo complexation reactions with silver(I) cations and that alkanes typically do not. Therefore, facilitated transport membranes including a silver(I) cationic carrier have been extensively researched at a laboratory scale with some success. Despite such extensive research, however, the use of facilitated transport membranes has not found industrial acceptance for olefin separation applications. This failure to gain industrial acceptance is largely due to problems associated with membrane and carrier stability in industrial settings.
For example, one class of membranes that have been proposed for alkene/alkane separations are the so-called immobilized liquid membranes. Immobilized liquid membranes involve a thin liquid film containing the ionic carrier. A major problem with the use of immobilized liquid membranes in industrial applications is that it is difficult to keep the liquid immobile and to keep the liquid of the membrane from evaporating. Another class of membranes that has been proposed are ion exchange membranes. With ion exchange membranes, the ionic carriers are contained within a polymeric material. A major problem with ion exchange membranes, however, is that they have been ineffective as transport membranes for alkenes unless the polymeric material is swollen with water. To address this problem, it is generally required to saturate the feed and permeate streams with water to prevent drying of the swollen membrane. Although such a procedure works well on a laboratory scale, it is impractical for most industrial applications. Immobilized liquid membranes and water-swollen ion exchange membranes, therefore, both suffer from a need to prevent liquid losses from the membranes during operation.
There is a significant need for improved membrane separation techniques to address the foregoing problems.